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Abstract:

A club shaft for an iron-type golf club is made of a fiber reinforced
resin. The weight W of the club shaft per 39 inches is not less than 55
grams and not more than 95 grams. The average bending rigidity EIa of the
club shaft is not more than 1.5 kgf sq.m. An expression (1)
EIa=<0.05W-1.25 is satisfied. Preferably, an expression (2)
EIa=<0.05W-1.75 is satisfied. More preferably, an expression (3)
EIa>=0.1W-6.5 is satisfied. Even if golfers are weak in flexing the
club shafts though the golf swings are powerful, the carry distance can
be increased.

Claims:

1. A club shaft for an iron-type golf club made of a fiber reinforced
resin, and having a per-39 inch weight W of not less than 55 grams and
not more than 95 grams, and an average bending rigidity EIa of not more
than 1.5 kgf sq.m, whereinan expression (1) EIa=<0.05W-1.25 is
satisfied.

2. The club shaft according to claim 1, whereinan expression (2)
EIa=<0.05W-1.75 is further satisfied.

3. The club shaft according to claim 1, whereinan expression (3)
EIa>=0.1W-6.5 is further satisfied.

4. An iron-type golf club comprisingthe club shaft according to claim 1,
andan iron-type golf club head attached to the tip end of the club shaft.

5. An iron-type golf club comprisingthe club shaft according to claim 2,
andan iron-type golf club head attached to the tip end of the club shaft.

6. An iron-type golf club comprisingthe club shaft according to claim 3,
andan iron-type golf club head attached to the tip end of the club shaft.

Description:

BACKGROUND OF THE INVENTION

[0001]The present invention relates to an iron-type golf club, more
particularly to a FRP shaft having a specific relation between the
bending rigidity and weight of the shaft.

[0002]An optimum flexure of the golf club shaft during golf swing can
accelerate the head speed just before impact and increase the dynamic
loft angle at the impact. Thus, the launch angle of the ball can be
optimized.

[0003]In the case of golfers with powerful right swings such as
professional golfers and advanced golfers, usually, the head speed is
high and the flexure during golf swing is large. Therefore, club shafts
with relatively heavy weight and less flexibility (stiff) are
conventionally used in iron-type golf clubs targeted at such users. On
the other hand, in the case of golfers with less powerful swings such as
beginners and senior golfers, usually, the head speed is relatively low
and the flexure is small. Therefore, club shafts with light weight and
high flexibility are conventionally used in iron-type golf clubs targeted
at such users. Thus, the club shafts of the conventional iron-type golf
clubs are generally categorized into the above-explained two groups.

[0004]In recent years, on the on other hand, FRP club shafts made of fiber
reinforced resins are widely used because the weight, bending rigidity
and the like can be controlled easily in comparison with metal club
shafts.

[0005]In the case that a golfer is powerful but the golf swing form is not
good, therefore, the club shaft can not be bent sufficiently during golf
swing, if the golfer uses the former type of a club shaft with relatively
heavy weight and less flexibility, then there is a tendency that the head
speed is not accelerated effectively because of the less flexure of the
club shaft. Further, there is a tendency that the golf club can not come
back to its right position at impact, thus the club face can not be
squared and as a result the directions of the hit balls become unstable
and the carry distance is decreased. If on other hand the golfer uses the
later type of a club shaft with light weight and high flexibility, then
there is a tendency that the golfer upsets the golf swing timing or golf
swing tempo, and the direction of the club face at impact becomes
unstable. As a result, the directions of the hit balls become unstable
and the carry distance is decreased.

SUMMARY OF THE INVENTION

[0006]It is therefore, an object of the present invention to provide an
iron-type golf club and a club shaft therefor, which is suitable for
powerful beginners and the like, and in which the directionality of the
hit ball become stable, and at the same time, the golf swing power can be
transferred to the club head at the maximum, without changing the current
golf swing tempo.

[0007]According to the present invention, a club shaft for an iron-type
golf club is made of a fiber reinforced resin, and has a per-39 inch
weight W of not less than 55 grams and not more than 95 grams, and an
average bending rigidity EIa of not more than 1.5 kgf sq.m, wherein an
expression (1) EIa=<0.05W-1.25 is satisfied.

[0008]Preferably, an expression (2) EIa=<0.05W-1.75 is satisfied.

[0009]More preferably, an expression (3) EIa>=0.1W-6.5 is satisfied.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a front view of an iron-type golf club according to the
present invention.

[0011]FIG. 2 is a diagram for explaining a method for measuring the
bending rigidity of the club shaft.

[0012]FIG. 3 is a graph showing the ranges defined by the expressions (1),
(2) and (3).

[0013]FIG. 4 shows a set of prepreg pieces which can be used to make the
club shaft.

[0014]FIG. 5 shows prepreg pieces of the club shafts used in the
undermentioned comparison tests.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015]Embodiments of the present invention will now be described in detail
in conjunction with accompanying drawings.

[0016]According to the present invention, golf club 1 is an iron-type golf
club such as first-ninth irons, sand-wedge, pitching-wedge,
approach-wedge and lob-wedge.

[0017]The golf club 1 comprise a club shaft 2, a club head 3 attached to
the tip end 2a, and a grip 4 attached to the butt end 2b.

[0018]The club head 3 is, in its principal part(s), made of at least one
kind of a metal material, e.g. stainless steels, aluminum alloys,
titanium alloys, pure titanium and the like. It is of course possible to
use a nonmetal material, e.g. fiber reinforced resins and the like in
order to form a part of the club head 3. The club head 1 can be either a
hollow structure or a solid structure. Preferably, the club head 1 has a
volume in a range of from 25 to 40 cc, and a weight in the range of from
220 to 300 grams.

[0019]The grip 4 can be a rubber grip, resin grip, leather grip or the
like.

[0020]The club shaft 2 is a circular tube made of a fiber reinforced resin
and tapered from a butt end 2b towards a tip end 2a.

[0021]As to the resin, thermosetting resins and thermoplastic resins can
be used.

[0024]As to the fiber reinforcement of the fiber reinforced resin,
inorganic fibers (carbon fibers, glass fibers, boron fibers, silicon
carbide fibers, alumina fibers and the like), organic fibers
(polyethylene fibers, polyamide fibers and the like), and metal fibers
can be used alone or in combination. Especially, those having a tensile
elastic modulus of from 3 to 50 tonF/sq.mm are preferred in view of the
strength and weight reduction.

[0025]The club shaft 2 has a length SL inch and a weight Wr grams, and the
value W (=Wr×39/SL) is preferably set in a range of not less than
55, more preferably not less than 60, but not more than 95, more
preferably not more than 80.

[0026]This value W expresses a weight of the club shaft converted into a
39-inch long shaft (hereinafter the "per-39 inch weight" W). Usually, the
length of the club shaft is changed according to specifications. e.g.
types of club head, loft angle, user's preference and the like.
Accordingly, it will be not significant to define the bending rigidity by
referring to such unspecified length. In this invention, therefore, based
on a new criterion, the weight of the club shaft converted into a
constant length of 39 inches, the weight and bending rigidity of the club
shaft are defied.

[0027]If the per-39 inch weight W is less than 55 grams, then the target
powerful users might feel very light in weight, and feel odd at the time
of address. As a result, the golf swing and the directionality of the hit
ball might become unstable.

[0028]If the per-39 inch weight W is more than 95 grams, then it becomes
difficult even for the target powerful users to swing the club head
through the ball, and there is a possibility that the swing speed and
carry distance are decreased.

[0029]For the similar reasons, the actual weight Wr is preferably set in a
range of not less than 40 grams, more preferably not less than 45 grams,
still more preferably not less than 50 grams, but not more than 105
grams, more preferably not more than 100 grams, still more preferably not
more than 95 grams.

[0030]If the actual club shaft length SL is too short, the length of the
club shaft can not be utilized to increase the head speed. If the actual
club shaft length SL is too long, it is difficult to swing the golf club
and as a result, there is a possibility that the head speed is decreased.

[0031]Therefore, the actual club shaft length SL is preferably set in a
range of not less than 500 mm, more preferably not less than 525 mm,
still more preferably not less than 550 mm, but not more than 1100 mm,
more preferably not more than 1075 mm, still more preferably not more
than 1050 mm.

[0032]The club shaft 2 has an average bending rigidity EIa in a range of
not more than 1.5 kgf sq.m, preferably not less than 1.7 kgf sq.m.

[0033]The average bending rigidity EIa is the mean value of the bending
rigidity EI measured at a plurality of positions 2P which are, as shown
in FIG. 2, set up along the club shaft 2 at a distance of 130 mm from the
tip end 2a and at intervals of 100 mm from this 130 mm position toward
the butt end 2b. Accordingly, the measuring position 2P most close to the
tip end 2a is at 130 mm therefrom. The distance between the butt end 2b
and the measuring position 2P most close to the butt end 2b depends on
the shaft length SL but it should be not less than 130 mm but less than
230 mm.

[0034]The bending rigidity EI at each of the positions 2P is measured with
a universal material testing machine through a three point bending test.
More specifically, as shown in FIG. 2, the club shaft 2 is supported by
two fulcrums J1 and J2 spaced apart from each other by 200 mm, so that
the central axis CL of the shaft is kept horizontally, and one of the
above-mentioned measuring positions 2P is positioned at the midpoint of
the two fulcrums J1 and J2. Then, an indenter P is let down onto the
measure position 2P from right above at a descending speed of 5
mm/minutes. When the load applied to the measure position 2P is increased
and reaches to 20 kgf, the indenter P is stopped. And the amount of
deflection of the club shaft 2 is measured at the position 2P.

[0035]From the amount of deflection, the bending rigidity EI at the
position 2P is computed, using the following expression:

Bending rigidity EI=load×(distance between J1 and J2)
3/(48×amount of deflection)

wherein the tips of the fulcrums J1 and J2 are rounded by a radius of 12.5
mm, the tip of the indenter is rounded by a radius 6.0 mm, the units of
the distance and the amount of deflection are "meter", and the unit of
the load (force) is "kgf".

[0036]If the average bending rigidity EIa is less than 1.5 kgf sq.m, as
the club shaft is largely bent during golf swing, it becomes difficult to
swing the golf club, and since the direction of the club face becomes
unstable, the directionality of the hit ball is liable to become worse.
Further, it is difficult to make a powerful shot. If on the other hand,
the average bending rigidity EIa is too large, the flexure of the club
shaft 2 during golf swing becomes insufficient, and the head speed and
dynamic loft angle at impact can not be increased effectively. As a
result, it becomes difficult to effectively increase the carry distance.

[0037]In consideration of these facts, the per-39 inch weight W and
average bending rigidity EIa are configured to satisfy the following
expression (1)

EIa=<0.05W-1.25, (1)

preferably to satisfy the following expression (2)

EIa=<0.05W-1.75, (2)

more preferably to satisfy the following expression (3)

EIa>=0.1W-6.5. (3)

[0038]FIG. 3 is a graph showing the ranges of the average bending rigidity
EIa and per-39 inch weight W defined by the expressions (1)-(3). In the
graph, an ellipse indicates a range in which conventional club shafts are
mapped. Such a design concept that when the per-39 inch weight W is
large, the average bending rigidity EI is also large, can be read into
this distribution map. As having been explained, if a club shaft with
such heavy weight and high rigidity is used by such a golfer that the
golf swings are powerful but swing forms are not good, the club shaft can
not be bent sufficiently during golf swing, therefore, it is difficult to
improve the carry distance and directionality. In the present invention,
as the per-39 inch weight W is limited in a specific range. In the
present invention, as the per-39 inch weight W is limited in a specific
range, the club shaft 2 is provided with an optimum weight by which it
becomes easy to address and get appropriate swing timing. Further, as the
upper limit for the average bending rigidity EIa is defined within a
specific range, a suitable flexure can be secured during golf swing.
Furthermore, as the expression (1) is satisfied, the average bending
rigidity EIa becomes relatively small for the club shaft weight in
comparison with the club shafts of the ordinary concept, therefore, the
golfer with powerful swings but no-good swing forms, can obtain a perfect
flexure, without the need to changing the previous golf swing timing, and
as a result, the directionality and carry distance of the hit ball can be
greatly improved.

[0039]It is preferable that, in addition to the above limitation of the
average bending rigidity EIa, the bending rigidity EI at each measuring
position 2P is limited as follows.

[0040]The bending rigidity EI can be expressed as a function EI(x) of a
distance "x" in millimeter of the measure position from the tip end 2a of
the club shaft 2, namely, El(130), El(230), El(330), El(430), El(530),
El(630), El(730), El(830), El(930), El(1030), El(1130) . . . .

[0041]As explained above, the preferable range of the shaft length SL is
500 to 1100 mm.

[0044]Thus, in the case that SL is 500 to 1100 mm, the number (m) of the
measuring points 2P becomes 3 to 9 depending on the shaft length SL.

[0045]The bending rigidity EI(130), EI(230) and EI(330) at the first,
second and third measuring points from the tip end 2a toward the butt end
2b are preferably set in a range of not less than 0.5 kgf sq.m, more
preferably not less than 0.6 kgf sq.m, still more preferably not less
than 0.7 kgf sq.m, but not more than 3.0 kgf sq.m, more preferably not
more than 2.8 kgf sq.m, still more preferably not more than 2.5 kgf sq.m.

[0046]If at least one of EI(130), EI(230) and EI(330) is less than 0.5 kgf
sq.m, the flexure of the tip end part of the club shaft becomes
excessive, and the durability is decreased. Further, the directionality
of the hit ball is liable to become unstable. If at least one of EI(130),
EI(230) and EI(330) is more than 3.0 kgf sq.m, the flexure of the tip end
part of the club shaft becomes insufficient, and it is difficult to
accelerate the head speed just before impact. Moreover, the hit feeling
is liable to become worse because the shock of hitting a ball transmitted
to the player's hands increases.

[0047]When the number (m) of the measuring points 2P is 7 or more, the
bending rigidity is expressed as follows: EI(130), EI(230), EI(330),
EI(n*100+30), EI{(m-2)*100+30)}, EI{(m-1)*100+30)}, and EI(m*100+30),
wherein "n" is an integer variable more than 3 and less than m-2. (when
m=7, n=4), (when m=9, n=4, 5 and 6)

[0048]If the bending rigidity EI(n*100+30) of the middle part of club
shaft 2 is less than 0.8 kgf sq.m, since the flexure of the shaft during
golf swing increases in the middle part, there is a tendency that the
user upsets the golf swing timing or golf swing tempo. If the bending
rigidity EI(n*100+30) is more than 6.5 kgf sq.m, as the flexure is
decreased, there is a possibility that the acceleration of the head speed
becomes insufficient, and the power of golf swing can not be transferred
to the club head effectively.

[0049]Therefore, the bending rigidity EI(n*100+30) is preferably set in a
range of not less than 0.8 kgf sq.m, more preferably not less than 0.9
kgf sq.m, still more preferably not less than 1.0 kgf sq.m, most
preferably not less than 1.1 kgf sq.m, but not more than 6.5 kgf sq.m,
more preferably not more than 6.0 kgf sq.m, still more preferably not
more than 5.5 kgf sq.m, most preferably not more than 5.0 kgf sq.m.

[0050]Further, the bending rigidity EI(n*100+30) is more than EI(130),
EI(230) and EI(330).

[0051]When the integer variable "n" include a plurality of integers, it is
preferred that the bending rigidity EI(n*100+30) gradually increases
towards the butt end 2b.

[0052]The bending rigidity EI{(m-2)*100+30)}, EI{(m-1)*100+30)} and
EI(m*100+30) at the last three measuring points on the butt end side are
preferably set in a range of from 1.5 to 8.5 kgf sq.m.

[0053]If at least one of these is less than 1.5 kgf sq.m, since the
flexure of the shaft during golf swing increases, there is a tendency
that the user upsets the golf swing timing or golf swing tempo. If more
than 8.5 kgf sq.m, it becomes difficult for the user to feel the flexure
of the club shaft 2 during golf swing, and thus difficult to swing the
golf club.

[0054]Further, it is preferred that the bending rigidity gradually
increases towards the butt end 2b, namely,
EI{(m-2)*100+30)}<EI{(m-1)*100+30)}<EI(m*100+30), and
EI{(m-2)*100+30)} is 1.5 to 7.0 kgf sq.m, EI{(m-1)*100+30)} is 1.8 to 8.0
kgf sq.m, EI(m*100+30) is 2.0 to 8.5 kgf sq.m.

[0055]The club shaft 2 is, as shown in FIG. 4, made from pieces S1 to S2
of prepreg (hereinafter, generically, the "prepreg pieces S").

[0056]The prepreg pieces S are wound around a mandrel (not shown) into a
laminated tube. The mandrel is removed from the tube. An inflatable
bladder (not shown) is inserted into the laminated tube. The laminated
tube is put in a mold (not shown) together with the bladder. By inflating
the bladder and applying heat, the laminated tube is cured and molded
into the FRP shaft 2.

[0057]FIG. 4 shows an example set of prepreg pieces S constituting a shaft
2, wherein the prepreg pieces S are wound in the order from the prepreg
piece shown on the top of the figure to that on the bottom. The prepreg
pieces S include: small tip-end-side prepreg pieces S1 forming a part
near the tip end 2a of the club shaft 2; and long prepreg pieces S2
continuous over the entire shaft length.

[0058]Aside from this example, various combinations of prepreg pieces are
possible. For example, a small butt-end-side prepreg piece forming a part
near the butt end 2b of the club shaft 2 can be used.

[0059]In order to control the rigidity and to increase the strength of the
tip end part of the club shaft 2, the number of layers formed by the
winded prepreg pieces S1 is preferably at least 2, but at most 20,
preferably not more than 19, more preferably not more than 18.

[0060]The orientation angle of the fiber reinforcements (f) in the
tip-end-side prepreg piece S1 can be set in a wide range of from 0 to 90
degrees with respect to the direction of the shaft axis. For example, if
it is desired to increase the bending rigidity EI near the tip end 2a, it
is preferred to set the orientation angle in a range of not more than 10
degrees, most preferably at 0 degree. If it is desired to increase the
torsional rigidity, it is preferred to set the orientation angle in a
range of from 40 to 50 degrees, most preferably at 45 degrees.

[0061]The shape of the tip-end-side prepreg piece S1 (the shape developed
on a plane) can be either a quadrangular shape S1a or a triangular shape
S1b.

[0062]In either case, it is preferable that a side on the butt end side
(and at least one of two lateral sides in the case of the quadrangular
shape S1a) is inclined at an angle of 30 to 60 degrees with respect to
the longitudinal direction of the shaft in order to avoid a formation of
a large step difference in rigidity.

[0063]The long prepreg pieces S2 determine the fundamental bending
rigidity and strength of the club shaft 2.

[0064]If the number of layers formed by the winded long prepreg pieces S2
is less than 5, there is a possibility that the club shaft 2 lacks
necessary rigidity and strength. If more than 20, not only the production
efficiency is lowered, but also the likelihood of getting voids between
the layers is increased. Therefore, the number of layers formed by the
winded long prepreg pieces S2 is set in a range of not less than 5,
preferably not less than 6, more preferably not less than 7, but not more
than 20, preferably not more than 19, more preferably not more than 18.

[0065]The long prepreg pieces S2 include: [0066]a bias piece S2a whose
fiber reinforcements (f) are oriented at an angle of not less than 10
degrees, preferably not less than. 20 degrees, but not more than 80
degrees, preferably not more than 70 degrees; [0067]a parallel piece S2b
whose fiber reinforcements (f) are oriented at an angle of substantially
0 degree; and optionally an orthogonal piece S2c whose fiber
reinforcements (f) are oriented at an angle of substantially 90 degrees,
[0068]wherein the angles are referred to with respect to the direction of
the shaft axis.

[0069]The bias piece S2a is most useful for increasing the torsional
rigidity. For that purpose, the number of layer formed by the winded bias
piece(s) S2a is set in a range of not less than 1, preferably not less
than 2, more preferably not less than 3, but not more than 12, preferably
not more than 11, more preferably not more than 10. Here, the term "not
less than 1" means that the bias piece S2a is winded completely at least
once around the shaft. It is preferable that the layers of the bias
piece(s) S2a include a pair of layers whose orientation angles are
directionally opposite with respect to the longitudinal direction of the
shaft, more preferably orientation angles are same in the absolute values
(for example 45 degrees with respect to the longitudinal direction).

[0070]The parallel piece S2b is most useful for increasing the bending
rigidity. For that purpose, the number of layers formed by the winded
parallel piece(s) S2b is set in a range of not less than 2, preferably
not less than 3, but not more than 10, preferably not more than 9, more
preferably not more than 8.

[0071]The use of the orthogonal piece S2c in combination with the bias
piece S2a and parallel piece S2b is very useful for increasing the crush
strength of the shaft.

[0072]If the club shaft is already provided with a sufficient crush
strength by the arrangement of the bias piece S2a and parallel piece S2b,
it is not always necessary to use the orthogonal piece S2c. It can be
omitted. Even when used, the number of layers formed by the winded
orthogonal piece(s) S2c is preferably set in a range of not more than 4,
more preferably not more than 3, still more preferably not more than 2,
in order to avoid an unwanted increase of the club shaft weight.

[0073]Aside from the above method, various methods, e.g. so called
filament winding method, sheet winding method and the like can be
employed.

Comparison Tests

[0074]FRP shafts were made using prepreg pieces and comparison tests were
carried out.

[0075]The prepreg pieces used are shown in FIG. 5.

[0076]The prepreg pieces A-G were winded around a mandrel from A to G.
Firstly, an intermediate of the club shaft having a length of about 1200
mm was made and then cut into the target length of 1000 mm by removing
both of the ends.

[0077]In order to change the average bending rigidity EIa, the tensile
elastic modulus of the fiber reinforcement and the number of layers
(winding number) of each prepreg piece were changed as shown in Table 1.

[0086]Golf clubs were made by attaching an iron-type golf club head and a
rubber grip to the tip end and butt end, respectively, of each of the
shafts.

[0087]The club head was made of a soft iron and having a loft angle of 32
degrees.

[0088]Comparison tests were carried out by ten right-handed golfers
(handicap 0 to 20, age 20 to 40). Each of the golfers hit golf balls (SRI
sport limited, "XXIO") ten times par each golf club. Thus, 100 shots (10
shots×10 golfers) were made per each club.

[0089]The head speed just before impact and the ball lunching angle of
each shot were measured to obtain the mean values of the 100 shots. The
results are shown in Table by an index based on Ex.1 being 100.

[0090]As to the head speed, the larger index number means that the head
speed was more accelerated by the flexure of the club shaft. As to the
launching angle, the larger index number means that the launching angle
was more increased by the optimal flexure of the club shaft.

[0091]Further, with respect to the ten shots made by each of the golfers
with each club, the difference between the direction of the trajectory of
the ball and the direction of the target trajectory was measured and the
standard deviation was computed. With respect to each of the clubs, the
mean value of the standard deviation obtained by the ten golfers was
computed. The results are indicated by an index based on Ex.1 being 100,
wherein the smaller the index number, the better the directionality.

[0092]Furthermore, with respect to each of the shafts, easiness of swing
was evaluated into five ranks as follows, based on the ten golfers'
feeling.

[0094]From the test results, it was confirmed that the head speed and
carry distance can be increased and the directionality can be improved.

[0095]As described above, in the FRP golf club shaft according to the
present invention, since the club shaft weight converted into 39-inch
club shaft length is set in a relatively heavy weight range, it is
possible for the golfers with powerful swings to swing the club without
upsetting the golf swing timing or golf swing tempo. Further, since the
average bending rigidity EIa of the club shaft is set in a specific
range, the club shaft is provided with a considerably small bending
rigidity for the per-39 inch weight, therefor, even for the golfers being
powerful but not good at flexing the shaft, it becomes possible to flex
the club shaft during golf swing, and as a result, the head speed and
carry distance can be increased. Further, it becomes easy to return the
club face to the proper address position and the directionality of the
hit ball can be improved.